Comparative Planetology of the Outer Planets

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Comparative Planetology of the Outer Planets
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• Chapter 18

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A Travel Guide to the Outer Planets
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• Hydrogen-rich atmospheres

• belt-zone circulation

• interiors mostly liquid hydrogen

• shallow atmospheres

• Large satellite systems

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Jupiter
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Largest and most massive planet in the solar
system
Contains almost 3/4 of all planetary matter in
the solar system.
Most striking features visible from Earth
Multi-colored cloud belts
Visual image
Infrared false-color image
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Jupiters Interior
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From radius and mass ? Average density of Jupiter
1.34 g/cm3
gt Jupiter can not be made mostly of rock, like
earthlike planets.
? Jupiter consists mostly of hydrogen and helium.
T 30,000 K
Due to the high pressure, hydrogen is compressed
into a liquid, and even metallic state.
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Aurorae on Jupiter
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Just like on Earth, Jupiters magnetosphere
produces aurorae concentrated in rings around the
magnetic poles.
1000 times more powerful than aurorae on Earth.
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Jupiters Magnetic Field
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Discovered through observations of decimeter
(radio) radiation
Magnetic field at least 10 times stronger than
Earths magnetic field.
Magnetosphere over 100 times larger than Earths
magnetosphere.
Extremely intense radiation belts
Very high energy particles can be trapped
radiation doses corresponding to 100 times
lethal doses for humans!
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Comet Impact on Jupiter
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Impacts occurred just behind the horizon as seen
from Earth, but came into view about 15 min.
later.
Impact of 21 fragments of comet Shoemaker-Levy 9
in 1994
Impact sites appeared very bright in the infrared.
Visual Impacts seen for many days as dark spots
Impacts released energies equivalent to a few
megatons of TNT (Hiroshima bomb 0.15 megaton)!
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Jupiters Atmosphere
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Jupiters liquid hydrogen ocean has no surface
Gradual transition from gaseous to liquid phases
as temperature and pressure combine to exceed the
critical point.
Jupiter shows limb darkening ? hydrogen
atmosphere above cloud layers.
Only very thin atmosphere above cloud layers
transition to liquid hydrogen zone 1000 km
below clouds.
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Clouds in Jupiters Atmosphere (II)
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Three layers of clouds
1. Ammonia (NH3) crystals
2. Ammonia hydrosulfide (NH4SH)
3. Water crystals
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The Cloud Belts of Jupiter
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Dark belts and bright zones.
Zones higher and cooler than belts high-pressure
regions of rising gas.
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The Cloud Belts on Jupiter (II)
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Just like on Earth, high-and low-pressure zones
are bounded by high-pressure winds.
Jupiters cloud belt structure has remained
unchanged since humans began mapping them.
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Jupiters Ring
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Not only Saturn, but all four gas giants have
rings.
Galileo spacecraft image of Jupiters ring,
illuminated from behind
Jupiters ring dark and reddish only discovered
by Voyager 1 spacecraft.
Composed of microscopic particles of rocky
material
Location Inside Roche limit, where larger bodies
(moons) would be destroyed by tidal forces.
Ring material cant be old because radiation
pressure and Jupiters magnetic field force dust
particles to spiral down into the planet.
Rings must be constantly re-supplied with new
dust.
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Jupiters Family of Moons
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Over two dozen moons known now new ones are
still being discovered.
Four largest moons already discovered by Galileo
The Galilean moons
Io
Europa
Ganymede
Callisto
Interesting and diverse individual geologies.
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Callisto The Ancient Face
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Tidally locked to Jupiter, like all of Jupiters
moons.
Av. density 1.79 g/cm3
? composition mixture of ice and rocks
Dark surface, heavily pocked with craters.
No metallic core Callisto never differentiated
to form core and mantle.
? No magnetic field.
Layer of liquid water, 10 km thick, 100 km
below surface, probably heated by radioactive
decay.
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Ganymede A Hidden Past
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Largest of the 4 Galilean moons.
Av. density 1.9 g/cm3

• Rocky core

• Ice-rich mantle

• Crust of ice

1/3 of surface old, dark, cratered
rest bright, young, grooved terrain
Bright terrain probably formed through flooding
when surface broke
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Europa A Hidden Ocean
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Av. density 3 g/cm3
? composition mostly rock and metal icy surface.
Close to Jupiter ? should be hit by many
meteoroid impacts but few craters visible.
? Active surface impact craters rapidly erased.
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The Surface of Europa
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Cracked surface and high albedo (reflectivity)
provide further evidence for geological activity.
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The Interior of Europa
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Europa is too small to retain its internal heat ?
Heating mostly from tidal interaction with
Jupiter.
Core not molten ? No magnetic field.
Europa has a liquid water ocean 15 km below the
icy surface.
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Io Bursting Energy
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Most active of all Galilean moons no impact
craters visible at all.
Over 100 active volcanoes!
Activity powered by tidal interactions with
Jupiter.
Av. density 3.55 g/cm3 ? Interior is mostly
rock.
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The History of Jupiter
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• Formed from cold gas in the outer solar nebula,
  where ices were able to condense.

• In the interior, hydrogen becomes metallic (very
  good electrical conductor)

• Rapid rotation ? strong magnetic field

• Rapid growth

• Rapid rotation and large size ? belt-zone cloud
  pattern

• Soon able to trap gas directly through gravity

• Heavy materials sink to the center

• Dust from meteorite impacts onto inner moons
  trapped to form ring

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Saturn
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Mass 1/3 of mass of Jupiter
Radius 16 smaller than Jupiter
Av. density 0.69 g/cm3 ? Would float in water!
Rotates about as fast as Jupiter, but is twice as
oblate ? No large core of heavy elements.
Mostly hydrogen and helium liquid hydrogen core.
Saturn radiates 1.8 times the energy received
from the sun.
Probably heated by liquid helium droplets falling
towards center.
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Saturns Atmosphere
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Three-layered cloud structure, just like on
Jupiter
Main difference to Jupiter
Fewer wind zones, but much stronger winds than on
Jupiter
Winds up to 500 m/s near the equator!
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Saturns Rings
A Ring
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B Ring
Ring consists of 3 main segments A, B, and C Ring
C Ring
separated by empty regions divisions
Cassini Division
Rings cant have been formed together with Saturn
because material would have been blown away by
particle stream from hot Saturn at time of
formation.
Rings must be replenished by fragments of passing
comets / meteoroids.
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Composition of Saturns Rings
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Rings are composed of ice particles
moving at large velocities around Saturn, but
small relative velocities (all moving in the same
direction).
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Shepherd Moons
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Some moons on orbits close to the rings focus the
ring material, keeping the rings confined.
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Divisions and Resonances
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Moons do not only serve as shepherds.
Where the orbital period of a moon is a
small-number fractional multiple (e.g., 23) of
the orbital period of material in the disk
(resonance), the material is cleared out
? Divisions
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Electromagnetic Phenomena in Saturns Rings
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Radial spokes in the rings rotate with the
rotation period of Saturn
Magnetized ring particles lifted out of the ring
plane and rotating along with the magnetic-field
structure
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Titan Saturns Largest Moon
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About the size of Jupiters moon Ganymede.
Rocky core, but also large amount of ice.
Thick atmosphere, hiding the surface from direct
view.
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Titans Atmosphere
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Because of the thick, hazy atmosphere, surface
features are only visible in infrared images.
Many of the organic compounds in Titans
atmosphere may have been precursors of life on
Earth.
Surface pressure 50 greater than air pressure
on Earth
Surface temperature 94 K (-290 oF)
? Methane and ethane are liquid!
Methane is gradually converted to ethane in the
Atmosphere
? Methane must be constantly replenished,
probably through breakdown of ammonia (NH3).
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Saturns Smaller Moons
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Saturns smaller moons formed of rock and ice
heavily cratered and appear geologically dead.
Tethys Heavily cratered marked by 3 km deep,
1500 km long crack.
Iapetus Leading (upper right) side darker than
rest of surface because of dark deposits.
Enceladus Possibly active regions with fewer
craters, containing parallel grooves, possibly
filled with frozen water.
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Uranus
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• 1/3 the diameter of Jupiter

• 1/20 the mass of Jupiter

• no liquid metallic hydrogen

• Deep hydrogen helium atmosphere

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The Motion of Uranus
Orbit slightly elliptical orbital period 84
years.
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Very unusual orientation of rotation axis Almost
in the orbital plane.
97.9o
19.18 AU
Possibly result of impact of a large planetesimal
during the phase of planet formation.
Large portions of the planet exposed to eternal
sunlight for many years, then complete darkness
for many years!
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The Atmosphere of Uranus
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Like other gas giants No surface.
Gradual transition from gas phase to fluid
interior.
Mostly H 15 He, a few methane, ammonia and
water vapor.
Optical view from Earth Blue color due to
methane, absorbing longer wavelengths
Cloud structures only visible after artificial
computer enhancement of optical images taken from
Voyager spacecraft.
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Cloud Structures of Uranus
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Hubble Space Telescope image of Uranus shows
cloud structures not present during Voyagers
passage in 1986.
? Possibly due to seasonal changes of the cloud
structures.
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The Rings of Uranus
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Rings of Uranus and Neptune are similar to
Jupiters rings.
Confined by shepherd moons consist of dark
material.
Rings of Uranus were discovered through
occultations of a background star
Apparent motion of star behind Uranus and rings
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The Rings of Neptune
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Interrupted between denser segments (arcs)
Made of dark material, visible in
forward-scattered light.
Ring material must be regularly re-supplied by
dust from meteorite impacts on the moons.
Focused by small shepherd moons embedded in the
ring structure.
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The Moons of Uranus
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5 largest moons visible from Earth.
10 more discovered by Voyager 2 more are still
being found.
Ariel
Dark surfaces, probably ice darkened by dust from
meteorite impacts.
5 largest moons all tidally locked to Uranus.
Miranda
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Neptune
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Discovered in 1846 at position predicted from
gravitational disturbances on Uranus orbit by J.
C. Adams and U. J. Leverrier.
Blue-green color from methane in the atmosphere
4 times Earths diameter 4 smaller than Uranus
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The Atmosphere of Neptune
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Cloud-belt structure with high-velocity winds
origin not well understood.
Darker cyclonic disturbances, similar to Great
Red Spot on Jupiter, but not long-lived.
White cloud features of methane ice crystals
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The Moons of Neptune
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Two moons (Triton and Nereid) visible from Earth
6 more discovered by Voyager 2
Unusual orbits
Triton Only satellite in the solar system
orbiting clockwise, i.e. backward.
Nereid Highly eccentric orbit very long orbital
period (359.4 d).
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The Surface of Triton
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Very low temperature (34.5 K)
? Triton can hold a tenuous atmosphere of
nitrogen and some methane 105 times less dense
than Earths atmosphere.
Surface composed of ices nitrogen, methane,
carbon monoxide, carbon dioxide.
Possibly cyclic nitrogen ice deposition and
re-vaporizing on Tritons south pole, similar to
CO2 ice polar cap cycles on Mars.
Dark smudges on the nitrogen ice surface,
probably due to methane rising from below
surface, forming carbon-rich deposits when
exposed to sunlight.
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The Surface of Triton (II)
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Ongoing surface activity Surface features
probably not more than 100 million years old.
Large basins might have been flooded multiple
times by liquids from the interior.
Ice equivalent of greenhouse effect may be one of
the heat sources for Tritons geological activity.
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Pluto as a Planet
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• Virtually no surface features visible from Earth.

• 65 of size of Earths Moon.

• Highly elliptical orbit coming occasionally
  closer to the sun than Neptune.

• Orbit highly inclined (17o) against other
  planets orbits

? Neptune and Pluto will never collide.

• Surface covered with nitrogen ice traces of
  frozen methane and carbon monoxide.

• Daytime temperature (50 K) enough to vaporize
  some N and CO to form a very tenuous atmosphere.

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Plutos Moon Charon
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Discovered in 1978 about half the size and 1/12
the mass of Pluto itself.
Tidally locked to Pluto.
Hubble Space Telescope image
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Pluto and Charon
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Orbit highly inclined against orbital plane.
From separation and orbital period Mpluto 0.2
Earth masses.
Density 2 g/cm3 (both Pluto and Charon)
? 35 ice and 65 rock.
Large orbital inclinations ? Large seasonal
changes on Pluto and Charon.
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The Origin of Pluto and Charon
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Probably very different history than neighboring
Jovian planets.
Pluto and Charon formed as moons of Neptune,
ejected by interaction with massive planetesimal.
Older theory
Mostly abandoned today since such interactions
are unlikely.
Modern theory Pluto and Charon members of Kuiper
belt of small, icy objects (see Chapter 25).
Collision between Pluto and Charon may have
caused the peculiar orbital patterns and large
inclination of Plutos rotation axis.